Hormones, probably. About 18 percent of women suffer from migraines, compared with just 6 percent of men. Doctors have proposed several explanations for the disparity, including different levels of external stress and gender-related differences in the psychological response to pain. The best research, however, now suggests that sex hormones are to blame, and loads of circumstantial evidence support this theory. In women, the headaches typically begin after puberty and tend to decrease in both frequency and intensity after menopause. (Migraines are one of the few neurological disorders to subside with age.) They are also more common during menstruation and less common during pregnancy. Intriguingly, doctors treating male-to-female transgendered people have noticed that after their patients begin hormone therapy, they start to experience migraines with the same frequency as genetic females.

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Although the research is still unsettled, there is growing scientific evidence that estrogen is the primary culprit. Migraines occur when inflammation around the brain triggers trigeminal nociceptors—cells responsible for pain sensations around the face—to transmit chemical signals. Nancy Berman and Kenneth McCarson, neurology researchers at the University of Kansas Medical Center, have shown that in mice these cells have estrogen receptors. Also, rats exposed to estrogen exhibit significantly worse migraine symptoms—except for nausea, since rats don't vomit—than their estrogen-starved counterparts at the same level of inflammation. At a biochemical level, they've documented changes indicating that pain signals are stronger when estrogen receptors are activated. Such evidence suggests the possibility that men might experience the inflammation associated with migraine just as often as women but their pain receptors have a less forceful response to it.

Charges of sexism have hovered around migraines for years. Advocates point out that the National Institutes of Health will spend $15 million on migraine research in 2011, less than 0.05 percent of its annual research budget. That's peanuts compared with what we spend on some other debilitating but usually nonfatal diseases like arthritis ($253 million) and sleep disorders ($230 million).